Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells use renewable fuels such as methanol; typical products from the electrochemical reactions are mostly carbon dioxide and water. Fuel cells can be an attractive alternative to the combustion of fossil fuels.
In the past, fuel cells used reformers to convert methanol into hydrogen gas for use by the fuel cells. Direct oxidation fuel cells offer considerable weight and volume advantage over the indirect reformer fuel cells. However, initial direct oxidation models used a strong acid electrolyte which caused corrosion, degradation of catalyst, and other problems that compromise efficiency. Problems associated with such conventional direct liquid-feed cells are well recognized in the art.
Jet Propulsion Laboratory (JPL) developed an improved direct liquid-feed cell using solid-state membrane electrolyte. The JPL fuel cell eliminates the use of liquid acidic and alkaline electrolyte and therefore solves many problems in the conventional fuel cells. The subject matter of this improvement is described in U.S. Pat. No. 5,599,638, U.S. patent application Ser. No. 08/569,452 filed Dec. 8, 1995, now U.S. Pat. No. 5,773,162 and U.S. patent application Ser. No. 08/827,319, filed Mar. 26, 1997, now U.S. Pat. No. 5,945,231 the disclosures of which are herewith incorporated by reference to the extent necessary for proper understanding.
FIG. 1 illustrates a typical structure 100 of a JPL fuel cell with an anode 120, a solid polymer proton-conducting cation-exchange electrolyte membrane 110, and a cathode 130 enclosed in housing 102. An anode 120 is formed on a first surface of the membrane 110 with a first catalyst for electro-oxidation and a cathode 130 is formed on a second surface thereof opposing the first surface with a second catalyst for electro-reduction. The anode 120, membrane 110, and the cathode 130 are hot press bonded together to form a composite multi-layer structure called the membrane electrode assembly (MEA). An electrical load 140 is connected to the anode 120 and cathode 130 for electrical power output.
The membrane 110 divides the fuel cell 100 into a first section 122 on the side of the anode 120 and a second section 132 on the side of the cathode 130. A feeding port 124 in the first section 122 is connected to a fuel feed system 126. A gas outlet 127 is deployed in the first section 122 to release gas therein and a liquid outlet 128 leads to a fuel re-circulation system 129 to recycle the fuel back to the fuel feed system 126. In the second section 132 of the cell 100, an air or oxygen supply 136 (e.g., an air compressor) supplies oxygen to the cathode 130 through a gas feed port 134. Water and used air/oxygen are expelled from the cell through an output port 138.
During operation, a mixture of an organic fuel (e.g., methanol) and water is fed into the first section 122 of the cell 100 while oxygen gas is fed into the second section 132. Electrochemical reactions happen simultaneously at both the anode 120 and cathode 130, thus generating electrical power. For example, when methanol is used as the fuel, the electro-oxidation of methanol at the anode 120 can be represented by: EQU Anode: CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.-
and the electro-reduction of oxygen at the cathode 130 can be represented by: EQU Cathode: O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O
Thus, the protons generated at the anode 120 traverse the membrane 110 to the cathode 130 and are consumed by the reduction reaction therein while the electrons generated at the anode 120 migrate to the cathode 130 through the electrical load 140. This generates an electrical current from the cathode 130 to the anode 120. The overall cell reaction is: EQU Cell: CH.sub.3 OH+1.5O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O
The energy generated by JPL's direct feed fuel cell and the advantages of using a solid electrolyte membrane fostered further research. Efforts are targeted toward improving manufacturing efficiency while achieving better performance at reduced cost.
Prior art for preparing methanol fuel cell's membrane electrode assembly, as disclosed in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452, involves the formation of catalyst layers on a porous carbon substrate which is then mounted on either side of a solid electrolyte membrane. Although considerable energy output has been achieved at high catalyst loading levels, there may be significant performance limitations associated with this process.
In some resulting catalyst layers, at least fifty percent of the catalyst gets impregnated deep in the pores of the carbon substrate. Hence, the impregnated catalyst are inaccessible for electrochemical reaction and are essentially wasted. Some prior art methods of preparing membrane electrode assemblies for direct methanol fuel cells employ excessive catalyst. Improved techniques of catalyst application may help reduce the amount of catalyst necessary for attaining the desired performance levels. Reduction of the use of expensive catalyst and more efficient catalyst utilization are improvements that may propel this technology toward commercialization.
Another obstacle to desired performance levels is inadequate catalyst/membrane interface. A large area of electrochemically active interface between the carbon-paper coated catalyst layer and the membrane is usually desired for attaining maximum energy output by a particular fuel cell. There are some prior art methods that rely on heat and pressure for membrane electrode assembly fabrication. Since the catalyst layer is kept relatively dry after application of the catalyst onto the carbon paper substrate, the interface formed between the catalyst layer and the membrane is usually not optimum. Improved methods to enhance the area of electrochemical contact at the catalyst layer/membrane interface are desired for attaining high performance levels.